In order to initiate combustion of an air/fuel mixture within an internal combustion engine, a spark ignition system generates a high voltage arc across the spark plug electrodes at the appropriate time in the engine operating cycle. The onset of the arc across the spark plug gap is timed to occur at a predetermined number of degrees of crankshaft rotation, usually before the piston has reached top dead center (TDC).
If the spark timing is properly set, the combustion process initiated by the spark plug action will cause a pressure increase to develop within the combustion chamber that will peak just shortly after TDC during the piston's power stroke. If the spark is initiated too late in the operating cycle (retarded timing), the pressure developed within the combustion chamber will not be efficiently converted by the engine into work. On the other hand, if the spark is initiated too early in the operating cycle (advanced timing), extremely high and potentially damaging pressures and temperatures may result. The pressure and temperature increases associated with advance timing are also not efficiently converted by the engine into a useful work output.
Excessive advanced timing can also lead to the occurrence of several other types of combustion chamber phenomena. One such phenomena is auto-ignition of the end gases and another is pre-ignition.
Auto-ignition is a condition where the end gases (the unburnt portion of the fuel-air mixture initially ignited by the movement of the flame front) explode spontaneously as a result of the cylinder temperature and pressure becoming too high for the type of fuel being burned in the engine. In response to the sudden release of energy, the cylinder temperature dramatically increases and the cylinder pressure fluctuates, alternately rising and falling, as a pressure wave travels back and forth across the combustion chamber. When caused by auto-ignition of the end gases, the rapid pressure and temperature fluctuations are seen to occur after TDC. If the rate at which energy is released through auto-ignition is high enough, the exploding gases will cause the cylinder walls to vibrate resulting in audible engine noises, including the distinctive sound known as "pinging".
Many engine developers believe that a mild degree of auto-ignition is desirable because it generates turbulence within the combustion chamber, which hastens the combustion process, at a critical time when the normal flame kernal is in the process of being quenched. Slight auto-ignition has also been found to reduce the amount of unburnt hydrocarbons remaining after the completion of the spark-triggered ignition process. By utilizing the energy released when the hydrocarbons are burned during mild auto-ignition, it follows that lower hydrocarbon emissions and improved fuel economy can be realized.
Because of the benefits stated above, among others, engine designers often seek to calibrate ignition systems so that the spark advance is close to the threshold of auto-ignition. However, excessive auto-ignition must be avoided since it leads to higher combustion chamber temperatures and is counter productive. In fact, these elevated temperatures can heat the spark plug electrodes to the point where they will initiate the combustion process independently of the occurrence of a spark. This phenomena is pre-ignition.
Pre-ignition, which can cause significant engine damage including perforation of the piston, is characterized by the occurrence of extremely high cylinder temperatures and pressures near TDC. The audible sound associated with pre-ignition is produced by the action of auto-ignition and, when extreme, referred to as "knock". Generally, it can be stated that auto-ignition leads to pre-ignition and, subsequently, that pre-ignition leads to further auto-ignition.
A number of factors influence the spark timing threshold which generates auto-ignition. Some of these factors include, inlet air temperature, engine speed, engine load, air/fuel ratio and fuel characteristics. Because accurate control of the spark timing is a significant contributor to engine performance, numerous types of engine control systems have been developed. These control systems typically employ a microprocessor based closed-loop spark timing control system which simultaneously measures a number of parameters, such as exhaust composition, coolant temperature, and the occurrence of spark knock via transducers. The resulting data is then processed to set the engine timing near a predicted auto-ignition threshold.
The knock detectors typically used in engine control systems are piezoelectric transducers which sense the intense vibration caused by spark knock. When used in the environment of an internal combustion engine, however, these transducers may not be selective enough to distinguish the slight vibration produced by incipient auto-ignition over the normal amount of engine vibration. For this reason, these detectors are typically not capable of sensing, particularly at high engine speeds, the threshold of auto-ignition. An engine control system is therefore needed which is capable of detecting incipient auto-ignition and which enables more precision in setting the spark timing in a closed-loop system.
Other characteristics found in ignition systems and considered to be undesirable include, but are not limited to: excessive spark plug electrode wear; the inability to fire fouled spark plugs; poor cold weather starting; poor exhaust emissions during cold engine starting and running; the remote generation of high voltages in the engine compartment by the ignition system; the routing and distribution of high voltages over considerable lengths of ignition wire; and the generation of significant mounts of electro-magnetic radiation within and around the ignition system, as well as the vehicle, during operation of the engine.
It is therefore an object of the present invention to provide an engine control and ignition system which overcomes the limitations and disadvantages of known systems.
It is also an object of this invention to provide an ignition system which is capable of performing various engine diagnostic procedures so as to operate as a feedback element of the engine control system. In particular, the invention operates as a non-invasive combustion chamber monitor through the utilization of the ignition transformer and the spark plug as the feedback elements.
The present invention has as further objects the providing of a method for determining engine load, a method for detecting engine misfire and a method for detecting auto-ignition of the end gases.
Another object of the invention is to provide a coil-on-plug ignition transformer which is capable of charging, tiring and retiring the spark plug at short, repeatable intervals as programmed into the engine control system.
One feature of this invention is that it eliminates the various problems associated with the distribution of high voltages throughout the ignition system. Another feature of the present invention is that it reduces the amount of electro-magnetic radiation generated by the ignition system around the engine and the vehicle itself.
Reduced spark plug electrode wear is another feature as well as the ability to fire badly fouled spark plugs.
A still further feature of the invention is enhanced cold weather starting capabilities of an internal combustion engine and the minimization of exhaust emissions which occur during cold starting and running. A related feature is the extension of the air/fuel ratio toward the lean limit which helps to further reduce emissions and improve fuel economy during normal engine operation.